Ion columns and Liquid Metal Ion Source LMIS methods are known and are employed in applications such as failure analysis, circuit edit and structural modification. The extraction region of an ion column includes beam limiting and extraction members, that function to both provide an electric potential to extract ions from the ion source, and to limit the diameter of the beam that passes through a hole defined substantially in the middle thereof. The ions blocked from passing impact the surface of the members and knock molecules free in a process called sputtering.
A common problem occurs when debris from the beam limiting and extraction members is back sputtered upon the source (LMIS). Debris back-sputtered onto the source causes instability in the LMIS ion emission. One solution to the back sputtered debris problem is described by Ward et al. in U.S. Pat. No. 5,034,612. Ward et al. constructs portions of the beam limiting and extraction apparatus using source friendly material. Source friendly material is defined as a material if attached to or impacted on the source has minimal impact on the source stability. Appropriate source friendly materials depend upon the composition of the source but can include, for example, W, Al, Cu, V, Nb, Ta, Re, Ti or its alloys.
Unfortunately no perfect source friendly material has been found. Some source friendly materials tend to have poor dimensional stability and are therefore inappropriate to use in the optical elements of the charged particle beam column. Some materials, although source friendly, actually have higher sputter rates, which cause more material to be back-sputtered onto the source. While proper selection of beam limiting and extraction materials helps extend the stability of the LMIS ion beam emitter, eventually the source will begin to fail. Source heat cycling can extend the life the life of a source. Heating the source causes contaminants to vaporize or sink below the surface of the liquid metal. Heating can be accomplished by passing a current through a filament which suspends the liquid metal source in place.
A prior art LMIS arrangement is shown generically in FIG. 1 by reference numeral 10. The source 12, suspended by an electrical filament 13, includes a needle shaped emitter 14 and a supply of liquid metal shown contained in a reservoir 16. The capacity of the reservoir 16 and the quantity of liquid metal is selected to ensure it does not become the source life limiter. The liquid metal runs from the reservoir 16 and down the emitter 14 in a thin film represented here by multiple drops 18. When the liquid metal reaches the end of the emitter 14, metal ions are extracted from the emitter and accelerated in a direction 20 toward a work piece by an extraction electrode 22. The ion current is controlled by the interaction between the flow of liquid metal atoms down the source 12 the electric potential of the extractor electrode or extraction cup 22, and the electric potential of a current control or suppression electrode 24. The ions leave the emitter 14 and form a beam 26. The beam 26 spreads as it leaves the source forming an emission cone 28 with the apex of the cone at tip of the emitter 14.
The beam 26 passes through a number of holes in the beam limiting and extraction members as the ions move toward the work piece 20. Each of these holes limit the outer envelope commonly referred to as the beam diameter. The beam 26 passes into the extraction cup 22 through a top hole 30. The shield 32 has a bottom hole 34 smaller than the extraction electrode top hole 30 which allows only the central portion of the beam 26 to pass. The portion of the beam 26 that passes through the shield bottom hole 34 impinges on a bottom plate 36 of the extraction cup 22.
The bottom plate contains a beam defining aperture or BDA 38. The term beam defining aperture (BDA) is usually used to describe the disk shaped element itself as well as the hole, or aperture 39 that passes through it. The aperture 39 in the BDA is significantly smaller than the other holes in the arrangement and consequently allows only small fraction of the original beam to pass through to the work piece. The majority of the beam impacts the shield 32, and the BDA 38.
When an ion beam impacts a surface with significant momentum, the molecules or atoms of the surface are consequently knocked free by a process commonly known as sputtering. Sputtered atoms are ejected in an oblong cloud with a central axis primarily in a direction near the normal to the surface. Since the surfaces impacted are perpendicular to the beam direction and source location, a high percentage of the sputtered debris is at risk of impacting the source. When sputtered material reaches the source it is described as back sputtered.
The shorter the distance from the BDA to the source the greater the probability of sputter impact on the source. However, increasing the distance reduces the column performance by reducing the resolution and/or beam current on the work piece.